This invention relates to a light beam scanning device using a rotating mirror, and more particularly to a light beam scanning device for use in a high speed information recording system connected with, for instance, an electronic computer. 2. Description of the Prior Art
In recent years, a number of devices have been developed for recording and reading out information by causing a beam of light such as a laser beam to scan an information recording medium. Some of these devices employ a rotating multi-face mirror as the beam deflecting means. No matter how precisely a rotating mirror is manufactured, there will inevitably be some degree of error in parallelism between the axis of rotation of the mirror and its reflecting surfaces. This error in parallelism causes the beam reflected by the mirror to be displaced in a direction perpendicular to that in which the mirror deflects the beam in the scanning operation. As a result, the scanning lines formed by the mirror on the scanning surface are not in perfect registration.
Several methods have been proposed for optically compensating for the displacement of the scanning lines so as to obtain scanning lines which register nearly perfectly on the scanning surface. For example, an invention for attaining such compensation is disclosed in Japanese Patent Application Public Disclosure No. 33642/1972. In this invention, the degree of compensation required to eliminate the deviation of the scanning line is first measured for each reflecting surface and the values obtained are stored in a memory device. During the scanning operation, signals are fed from the memory device to a separately provided beam deflecting means in synchronism with the rotation of the rotating multi-face mirror. The separately provided beam deflecting means is driven by the signals from the memory device so as to compensate for the error in parallelism and eliminate the displacement of the scanning lines. This system is disadvantageous in that it requires the provision of a second beam deflecting means and other complex devices for driving said means in accordance with the signals from the memory device.
On the other hand, Japanese Patent Application Public Disclosure No. 49315/1973 describes a system for eliminating scanning line deviation by an optical system employing two cylindrical lenses. The first of these cylindrical lenses is used to form on the reflecting surface of the rotating mirror a line image which lies parallel to the plane formed by the deflected light beams. The second cylindrical lens serves to put the point of deflection on the rotating mirror and the scanning surface into the relationship of object point and image point and as a result the deviation caused by the error in parallelism is eliminated. This system has several defects. Not only does it require two cylindrical lenses but these lenses must be provided in a specific relationship which is by no means easy to attain. Furthermore, as the beam incident on the rotating mirror lies in the same plane as the deflected beams, a large angle of beam deflection can be obtained only if the angle of incidence of the beam impinging on the rotating mirror is made large. As a result, the beam incident on the reflecting surface is apt to be eclipsed thereby.